Global ocean circulation during 1992–1997, estimated from ocean observations and a general circulation model

Abstract

[1] A three-dimensional oceanic state is estimated for the period 1992-1997 as it results from combining large-scale ocean data sets with a general circulation model. At the cost of increased computational load, the estimation (assimilation) method is chosen specifically so that the resulting state estimate is consistent with the model equations, having no artificial sources or sinks. To bring the model into close agreement with observations, its initial temperature and salinity conditions are permitted to change, as are the time-dependent surface fluxes of momentum, heat and freshwater. Resulting changes of these ''control vectors'' are largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses. The assimilation procedure is able to correct for many of the traditional shortcomings of the flow field by changing the surface boundary conditions. Changes in the resulting flow field are predominantly on the gyre scale and affect many features that are often poorly simulated in traditional numerical simulations, such as the strengths of the Gulf Stream and its extension, the Azores Current and the anticyclonic circulation associated with the Labrador Sea. Tests of the results and their consistency with prior error assumptions show that the constrained model has moved considerably closer to the observations imposed as constraints, but has also moved closer to independent data from the World Ocean Circulation Experiment not used in the assimilation procedure. In some regions where the comparisons remain indeterminate, not enough ocean observations are available, and it is difficult to ascribe the residuals to either the model or the observations. Although problems remain, a useful first solution to the global time-dependent ocean state estimation problem has been found. The estimates will continue to improve through the evolution of numerical models, computer power increases, more data, and more efficient estimation methods.

Transoceanic drift and the domestication of African bottle gourds in the Americas

“…To test the plausibility of oceanic dispersal on a scale sufficient to explain this level of diversity, we conducted a series of oceanic drift simulations based on wind values from the National Center for Environmental Protection/ National Center for Atmospheric Research, and surface current data from the Estimating the Circulation and Climate of the Ocean consortium (23,24). Both datasets constitute the best fit of numerical model output to empirical observations, and are used to calibrate a model that simulates drifting object displacement over time based on currents and winds.…”

Impact of atmosphere and sub‐surface ocean data on decadal climate prediction

“…However, this needs confirming with further experiments using pseudo observations sampled at actual Argo locations. We note that more sophisticated assimilation methods [e.g., Stammer et al, 2002;Zhang et al, 2007] could potentially produce additional skill to that of the simple nudging technique used in this work.…”

Development of a four‐dimensional variational coupled data assimilation system for enhanced analysis and prediction of seasonal to interannual climate variations

“…The ocean reanalysis data set was also used to create the initial conditions for the first-guess fields. The error covariance matrices for the observational data were assumed to be diagonal and were determined from the temporal changes of the corresponding variables in each model grid point [Stammer et al, 2002;Masuda et al, 2003]. Although the background terms for initial conditions are not involved explicitly in the cost function, a sequential input of the ocean reanalysis data set as an assimilation element mentioned above is expected to help maintain the covariance structure because of use of the same OGCM.…”

“…The success of these experiments likely reflects the fact that these studies benefited from the use of prescribed air-sea boundary conditions (e.g., sea surface temperature for atmospheric GCMs and air-sea fluxes for oceanic GCMs), although the Estimating the Circulation and Climate of the Ocean (ECCO) Consortium has successfully made long-term ocean reanalysis experiments by adjusting air-sea fluxes (and initial conditions) with a four-dimensional variational (4D-VAR) assimilation approach [e.g., Stammer et al, 2002].…”